set-GID for collaboration sticky bit

additional permission

1. set user id
2. set group id
3. sticky bit

SGID - group ownership of directory is inherited to items created inside
sticky bit - user can delete only file created by himself or user is owner of directory
with sticky bit you can remove file if you are
a) owner of directory
b) owner of file
===========
usermod lisa -aG sales
usermod lisa -G sales - will override secondary group assignments

===========

[linda@localhost sales]$ ls -lart

total 0

drwxr-xr-x. 3 root  root  19 Oct 15 09:29 ..

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda1

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda3

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda2

drwxrwx---. 2 root  sales 48 Oct 15 09:32 .

[linda@localhost sales]$ 

===========

user lisa from the same group sales cannot modify files created by user linda
===========
[root@localhost test]# chmod g+s sales/
[root@localhost test]# ls -lart
total 4
dr-xr-xr-x. 25 root root  4096 Oct 14 16:34 ..
drwxr-xr-x.  3 root root    19 Oct 15 09:29 .
drwxrws---.  2 root sales   48 Oct 15 09:32 sales
[root@localhost test]#

============

[root@localhost test]# cd sales/;touch root{1..2}

[root@localhost sales]# ls -lart

total 0

drwxr-xr-x. 3 root  root  19 Oct 15 09:29 ..

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda1

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda3

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda2

-rw-r--r--. 1 root  sales  0 Oct 15 09:34 root2

-rw-r--r--. 1 root  sales  0 Oct 15 09:34 root1

drwxrws---. 2 root  sales 74 Oct 15 09:34 .

[root@localhost sales]# 

=============

[root@localhost sales]# su - linda

Last login: Sun Oct 15 09:31:44 EDT 2017 on pts/0

[linda@localhost ~]$ cd /test/sales/

[linda@localhost sales]$ touch linda4

[linda@localhost sales]$ ls -lart

total 0

drwxr-xr-x. 3 root  root  19 Oct 15 09:29 ..

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda1

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda3

-rw-rw-r--. 1 linda linda  0 Oct 15 09:32 linda2

-rw-r--r--. 1 root  sales  0 Oct 15 09:34 root1

-rw-rw-r--. 1 linda sales  0 Oct 15 09:37 linda4

drwxrws---. 2 root  sales 75 Oct 15 09:37 .

[linda@localhost sales]$ ls -d

.

[linda@localhost sales]$ ls -ladrt

drwxrws---. 2 root sales 75 Oct 15 09:37 .

[linda@localhost sales]$ 

=============

lisa can delete linda's files because lisa is member of group sales which has write permissions on  directory

[linda@localhost sales]$ su - lisa
Password:
[lisa@localhost ~]$ cd /test/sales/
[lisa@localhost sales]$ rm -f linda*
[lisa@localhost sales]$ ls -lart
total 0
drwxr-xr-x. 3 root root  19 Oct 15 09:29 ..
-rw-r--r--. 1 root sales  0 Oct 15 09:34 root1
drwxrws---. 2 root sales 19 Oct 15 09:38 .
[lisa@localhost sales]$
=============
in order to prevent delete linda's files by lisa, we need sticky bit
=============
[root@localhost sales]# chmod +t .
[root@localhost sales]# ls -lartd
drwxrws--T. 2 root sales 19 Oct 15 09:38 .
[root@localhost sales]#

owner of the sales directory is root, not sales group

[lisa@localhost sales]$ echo text >> linda1

[lisa@localhost sales]$ cat linda1

text

[lisa@localhost sales]$ rm -f linda1

rm: cannot remove ‘linda1’: Operation not permitted